Radio frequency (RF)- and microwave-based sensing techniques, including cavity perturbation methods and the like, are used in many applications ranging from laboratory and research instrumentation to process control systems and even on-vehicle sensors. In many applications, antennas or probes are used to transmit and/or receive radio frequency or microwave signals to conduct these measurements.
Many conventional RF antennas are not robust and are incapable of surviving extended operation with exposure to high temperatures, high vibration levels, thermal shock, corrosive or dirty environments, oxidizing or reducing conditions, and the like. Environmental exposure, rain, snow, and salt water for example, as well as exposure to chemicals and solvents, such as oils, fuels, acids, and similar chemicals, is also detrimental to many conventional RF/microwave probes and antennas.
The RF antenna described herein may be used in a number of applications, ranging from cavities to transmission lines, and even in free space. One range of applications include systems which monitor changes in the dielectric properties of a material or a mixture of materials in order to deduce some information regarding the state of the system.
One specific example includes a class of radio-frequency measurement systems applied to monitor and sense vehicle exhaust emissions or the state of various vehicle emission control and sensor devices. Radio-frequency or microwave systems used to monitor the loading state of vehicle particulate filters, such as the amount of soot or ash accumulated in a diesel particulate filter, is one exemplary application. Another application includes the monitoring of various gaseous species, such as oxygen or oxides of nitrogen (among others), adsorbed onto various catalytic emission after treatment components, such as three-way catalytic converters, selective catalytic reduction systems, oxidation catalysts, or lean NOx traps, to name a few. In yet another embodiment, the monitored parameter may be a change in the dielectric properties of the material itself, such as the filter material in the case of a diesel particulate filter, in one example and the catalysts substrate, wash coat, or catalyst material in yet another example. Although after treatment filters and catalysts are described and are particularly challenging, any filter system or catalyst system can use the technology described herein.
The present invention is directed to an improved and lower cost radio frequency antenna or measurement probe, and method of making the same, with granular or powder insulating material and suitable for extended operation over a range of conditions, including exposure to high temperatures in excess of 900 degrees Celsius, vibration, mechanical stresses, water and other liquids, and the like in, for example, a radio frequency vehicle exhaust control or sensor system.